This invention relates to optical fiber cables.
We currently manufacture an optical fiber cable particularly for submarine telecommunication links and a cross section of this cable is shown in FIG. 1 of the accompanying drawings. It comprises an optical fiber package A loosely housed in a bore D within a strain member tube formed by a closed conductive C-section E surrounded by high tensile steel wires G, optionally surrounded by a copper tape H hermetically sealed at G.sup.1. Extruded around the wire layer G is a polythene dielectric layer J and for shallow waters an armoring K is applied, although for deep water this is unnecessary. There can be two layers of wires G and the tube E is preferably hermetically sealed by welding.
The optical fiber package A comprises several optical fibers B with primary and secondary coatings disposed around a plastic-coated king wire A.sup.1 and held together by a fiber whipping C (such as "Kevlar").
This cable is expensive in terms of fiber channels since only eight optical fibers are housed in it. With the advent of repeaterless links any constraint upon the number of fibers imposed by the repeater will disappear and it would be more economical to have a larger number of fibers in such a cable.
So-called acrylate-coated fibers are now available which have a smaller diameter than their secondary-clad counterparts and we have attempted to modify our existing fiber package to incorporate these fibers. We have incorporated the fibers into a package in a number of different ways, but without success. For example, we have bound a number of acrylate coated fibers directly onto a copper-coated high tensile steel kingwire 0.7mm diameter, using a Kevlar whipping but found that the loss of the fibers above the 1300 nm window was excessive, due mainly to microbending. We therefore increased the buffering between the kingwire and the fibers. We tried a plastic coated kingwire (0.5 mm kingwire coated to 0.7 mm) which showed little improvement, and so we tried a heavily buffered kingwire 0.2 diameter buffered with a softer plastic to 0.7 mm, but still with unacceptably high microbending losses, particularly above the 1300 nm window, e.g. 1550 nm.
We therefore considered alternative package structures. For example, British patent specification No. 2136350A discloses an undersea communications cable core in which a central strength member is heated and a first layer of elastomer is extruded onto the heated strength member. Then optical fibers are laid along a helical path onto the first layer of elastomer with a planetary motion. A second layer of elastomer is extruded over the fibers and merges with the first layer. A protective layer sheath is extruded around the second layer. This is said to minimize microbending losses with respect to the sea bottom pressure. However, it is a complicated and expensive manufacturing process, and it is believed that microbending losses, particularly during storage will still be significant at the longer wavelength mentioned above.